1. Field of the Invention
This invention relates to centrifugal pumps and let-down turbines and, more specifically, to high-temperature, high-pressure, high-volumetric flow rate centrifugal pumps and let-down turbines for the chemical process and power generating industries.
2. Description of the Prior Art
Coal liquefaction processes presently being considered for commercial development will require a high-capacity and high-pressure coal slurry feed system in the process. A scale-up of present-type reciprocating feed pumps to meet the high capacity will require a large number of units with attendant high capital and maintenance costs. As an alternate approach, high-volume centrifugal pumps appear promising in meeting the high-capacity requirement of the coal liquefaction plant. However, centrifugal pumps must be designed for high speeds to meet the high-pressure requirements, and may be subject to excessive internal wear because of high slurry velocities.
A comprehensive state-of-the-art study revealed that the centrifugal pump type most commonly used for pumping liquid/solid mixtures was found to be a low-speed, low-pressure, single-stage unit. In some applications, several of these units have been put in series to obtain high pressures.
For coarse slurry service, with solid sizes up to 21/2 inches, the pump impellers are designed for wide vane passage width and thick sections. The impeller is generally shrouded and operates with a large running clearance with the casing. To reduce abrasive wear in the clearance and to minimize leakage or accumulation of solid particles, both the impeller front and back shrouds have cast-in, expeller vanes to reduce recirculation. Both, hard metals such as Ni-hard alloy, and soft rubber-lined cast iron are commonly used. For typical mining or dredging services, the pumps develop 50 to 75 psi with the impellers operating from 60 to 85 feet/sec tip speed. For a given impeller tip speed needed for developing head, there is a preference of using a larger diameter impeller running at a lower rpm to obtain low slurry flow velocity and long pump wear life.
For fine slurry service, with typical solid sizes of 10 to 325 mesh, higher speed, single-stage chemical processing pumps have been used. The impellers are shrouded, the operating clearance with the casing is closer, and both the wear ring and seals are flushed with clean fluids. Commonly used materials are austenitic stainless steel, Ni-hard, white iron, and high chrome steel. For combined corrosion and erosion resistance, a silicon carbide liner has been used. The slurry concentrations in chemical processing range from approximately 5 to 15% by weight; however, these same pumps have been used for coal/oil slurries up to 40% concentration (e.g., SRC Plant, Wilsonville). These pumps develop pressures on the order of 150 psi and operate at an impeller tip speed of 140 feet/sec.
The centrifugal slurry pump of the present invention is designed to function as a hydraulic slurry let-down turbine by reversing the flow through the pump. The turbine converts high-pressure fluid into a lower pressure state thereby converting kinetic fluid energy into rotational horsepower. As pressure decreases within the turbine, fluid contained gases are released and drawn out of the system by any appropriate means.
The hydraulic turbine can be used in any process where a regular flow control valve is used to reduce (let down) high-pressure flow to lower pressures. Therefore, a hydraulic turbine can replace high-pressure let-down valves currently used in the coal liquefaction system or in the geopressure-geothermal energy recovery system. In the coal liquefaction system, let-down valves are used to reduce the high-pressure, high-temperature liquid coal in the reactor to a lower pressure liquid coal in the filter feed tank for further processing in a vacuum tower (to obtain solid refined coal) or in a fractionator (to obtain light distillate fuel or fuel oil.) In the geopressure-geothermal energy recovery system, the let-down valve is used directly at the wellhead to recover dissolved methane from high-pressure brine in geopressure reservoirs.
In using the centrifugal slurry pump as a hydraulic slurry let-down turbine, replacing the high-pressure let-down valves, two important features are achieved: (1) the severe erosion problem in valves is eliminated by the gradual changes in pressure and velocities achieved through the hydraulic turbine, and (2) the potential energy of the high-pressure flow is recovered as useful shaft power by the hydraulic turbine. Moreover, the longer operating life and energy recovery of the hydraulic turbine will result in lower life cycle cost over the let-down valve.
Typical state-of-the-art slurry pump and let-down turbine problems include: (a) housing and impeller (turbine rotor) erosion which has been partially solved by increasing wall thickness, hardening surfaces, applying resistant coatings, maintaining low velocities, having no discontinuities, and having replaceable liners; (b) compacting and grinding clearance problems have been partially solved by enlarging clearances and installing expellers; (c) seal wear problems have been partially solved by expellers, flingers and fluid flush means; (d) impingement and abrasion on the tongue have been partially solved by hardening surfaces, maintaining low fluid velocity and having resilient coatings; and (e) excessive wear of wear rings has been partially solved by fluid flush means and wide clearances.